2021-01-08T16:08:42Z
Acceptance in OA@INAF
þÿUsing [C II] 158 ¼m Emission from Isolated ISM Phases as a Star Formation Rate
Indicator
Title
Sutter, Jessica; Dale, Daniel A.; Croxall, Kevin V.; Pelligrini, Eric W.; Smith, J. D.
T.; et al.
Authors
10.3847/1538-4357/ab4da5
DOI
http://hdl.handle.net/20.500.12386/29624
Handle
THE ASTROPHYSICAL JOURNAL
Journal
886
Using
[C
II
]158 μm Emission from Isolated ISM Phases as a Star Formation Rate
Indicator
Jessica Sutter1 , Daniel A. Dale1 , Kevin V. Croxall2 , Eric W. Pelligrini3, J. D. T. Smith4 , Philip N. Appleton5 , Pedro Beirão6, Alberto D. Bolatto7 , Daniela Calzetti8 , Alison Crocker9 , Ilse De Looze10, Bruce Draine11 ,
Maud Galametz12 , Brent A. Groves13 , George Helou5, Rodrigo Herrera-Camus14 , Leslie K. Hunt15 , Robert C. Kennicutt16,17, Hélène Roussel18, and Mark G. Wolfire4
1Department of Physics & Astronomy, University of Wyoming, Laramie, WY, USA;jsutter4@uwyo.edu 2
Expeed Software, Columbus, OH, USA 3
Institute for Theoretical Astrophysics Heidelberg, Germany 4
Department of Physics & Astronomy, University of Toledo, Toledo, OH, USA 5
IPAC, California Institute of Technology, Pasadena, CA, USA 6
Machine Learning Company, Oss, The Netherlands 7
Department of Astronomy, University of Maryland, College Park, MD, USA 8
Department of Astronomy, University of Massachusetts, Amherst, MA, USA 9
Physics Department, Reed College, Portland, OR, USA 10
Department of Physics & Astronomy, University College London, London, UK 11
Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA
12Laboratoire AIM, CEA, Université Paris Diderot, IRFU/Service d’Astrophysique, Gif-sur-Yvette, France 13
Research School of Astronomy & Astrophysics, Australian National University, Canberra, Australia 14
Departamento de Astronomía, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Concepción, Chile 15
INAF—Osservatorio Astrofisico di Arcetri, Firenze, Italy 16
Steward Observatory, University of Arizona, Tucson, AZ, USA 17
Department of Physics & Astronomy, Texas A&M University, College Station, TX, USA 18
Institut d’Astrophysique de Paris, Sorbonne Universités, Paris, France
Received 2019 March 15; revised 2019 October 1; accepted 2019 October 10; published 2019 November 20
Abstract
The brightest observed emission line in many star-forming galaxies is the[C II]158μm line, making it detectable up to z∼7. In order to better understand and quantify the [C II emission as a tracer of star formation, the] theoretical ratio between the [N II]205μm emission and the [C II]158μm emission has been employed to empirically determine the fraction of [C II emission that originates from the ionized and neutral phases of the] interstellar medium(ISM). Sub-kiloparsec measurements of the [C II]158μm and [N II]205μm lines in nearby galaxies have recently become available as part of the Key Insights in Nearby Galaxies: a Far Infrared Survey with Herschel (KINGFISH) and Beyond the Peak programs. With the information from these two far-infrared lines along with the multi-wavelength suite of KINGFISH data, a calibration of the [C II emission line as a star] formation rate(SFR) indicator and a better understanding of the [C II deficit are pursued. [] C II emission is also] compared to polycyclic aromatic hydrocarbon(PAH) emission in these regions to compare photoelectric heating from PAH molecules to cooling by[C II in the neutral and ionized phases of the ISM. We] find that the [C II] emission originating in the neutral phase of the ISM does not exhibit a deficit with respect to the infrared luminosity and is therefore preferred over the [C II emission originating in the ionized phase of the ISM as an SFR] indicator for the normal star-forming galaxies included in this sample.
Unified Astronomy Thesaurus concepts:Galactic and extragalactic astronomy(563);Far infrared astronomy(529);
Interstellar medium(847);Photodissociation regions(1223);Star forming regions(1565)
1. Introduction
The[C II]158μm line is frequently the brightest observed emission line in star-forming galaxies (Luhman et al. 2003; Brauher et al.2008). This prominence is due to the prevalence
of carbon, its dominant role in cooling neutral atomic gas (Wolfire et al.2003), its sub-Rydberg ionization potential, and
the minimal dust attenuation it undergoes given its long wavelength(Heiles1994; Luhman et al.1998). The brightness
of this line makes[C II]158μm emission an invaluable tool for probing the interstellar medium (ISM) of remote galaxies and the fainter regions within the disks of nearby galaxies. As [C II]158μm is a prominent and typically unattenuated emission line, it has naturally become a target of interest for tracing physical properties such as star formation rate (SFR). [C II emission is expected to trace star formation because [] C II] is a primary coolant of the photodissociation regions (PDRs),
meaning cooling by [C II emission should balance photo-] electric heating from young, hot stars to maintain thermal stability. However, the relatively low ionization potential of neutral carbon (11.3 eV) complicates the potential diagnostic capabilities of this line. Due to its low ionization potential, C+ can exist within both the ionized and neutral ISM, including ISM phases spanning H II regions, warm ionized gas, cold atomic gas, and PDRs, which affects the interpretive power of the 158μm emission line as a tracer of any specific galactic property(Stacey et al.1985; Shibai et al.1991; Bennett et al.
1994; Pineda et al.2013).
This multiphase origin also affects [C II]’s potential as an SFR indicator through the effect known as the “[C II deficit”] (Malhotra et al.2001; Herrera-Camus et al.2015; Croxall et al.
2017). The [C II deficit is the decreasing trend in the ratio] of[C II]158μm luminosity to total infrared (TIR) luminosity © 2019. The American Astronomical Society. All rights reserved.
(i.e., the total luminosity over 5–1100 μm) with respect to various measures of luminosity or star-forming activity. Multiple studies of [C II]/TIR have found a decrease in this ratio as a function of increasing SFR surface density (Smith et al.2017), infrared luminosity (Malhotra et al.2001; Luhman et al.2003), far-infrared color (Helou et al.2001; Croxall et al.
2012; Parkin et al.2013; Díaz-Santos et al.2017), and the ratio
of infrared luminosity to H2gas mass(L MIR H2) (Graciá-Carpio et al.2011; Herrera-Camus et al.2018a). Several mechanisms
have been proposed and investigated to explain the observed relative decrease in[C II emission. These include:] (1) [C II is] optically thick or absorbed by dust in galaxies with the highest infrared luminosities (Abel et al. 2007; Neri et al. 2014), (2)
that activity of active galactic nuclei (AGNs) in the host galaxies could produce higher infrared luminosity and lower [C II emission due to the increased hardness of radiation in] AGN host galaxies potentially changing the ionization states of carbon in the ISM(Langer & Pineda2015; Herrera-Camus et al.
2018b), (3) the thermalization and saturation of the [C II line in] warm, high-density environments leads to the [O I]63μm line becoming the dominant cooling line in these regions (Muñoz & Oh 2016; Díaz-Santos et al.2017), (4) in regions with high
ionization parameters, a majority of the far-UV(FUV) radiation from young stars is absorbed within the H II regions rather than escaping to PDRs where[C II is the primary coolant] (Abel et al.
2009; Graciá-Carpio et al. 2011), or (5) that very small dust
grains in the most infrared-luminous galaxies become highly charged by increased FUV emission from star formation, increasing the energy needed to photoeject additional electrons and thus reducing the number and energy of photoejected electrons per unit FUV radiation (Malhotra et al.2001; Graciá-Carpio et al.2011). The [C II deficit has been measured across a] wide range of galaxy samples, from low-metallicity dwarf galaxies (Cormier et al. 2019), to ultra/luminous infrared
galaxies (U/LIRGS) with infrared luminosities above 1011Le (Díaz-Santos et al. 2017), to normal star-forming galaxies
(Malhotra et al. 2001; Smith et al. 2017). These studies of
multiplefine-structure cooling lines in a variety of galaxies and others like them have indicated that the third, fourth, and fifth explanations seem to be the most prominent causes of this deficit, although all five may play some part in creating the observed decline in the ratio of [C II] to TIR luminosity (Malhotra et al.2001; Luhman et al.2003; Croxall et al. 2012; Díaz-Santos et al.2017; Smith et al.2017).
Despite these difficulties, multiple groups have attempted to use [C II alone as an SFR indicator] (e.g., Stacey et al.1991; Boselli et al.2002; De Looze et al.2011; Herrera-Camus et al.
2015). Although there are indications that [C II emission] primarily originates in and around star-forming regions(Stacey et al. 1985; Mookerjea et al. 2011), the relationships found
between [C II luminosity and other tracers of star formation] such as extinction-corrected Hα (Boselli et al.2002) and FUV
luminosity(De Looze et al.2011) often show a large scatter (as
large as a factor of 10) and do a poor job of matching SFRs in extreme cases (Sargsyan et al. 2012; De Looze et al. 2014; Díaz-Santos et al. 2017). However, it has been found that by
including additional spectral information it is possible to determine a better constrained [C II]–SFR relationship. For example, using [C II surface brightness along with an infrared] color correction can predict the SFR to within a factor of three for all but the most IR-luminous systems(Herrera-Camus et al.
2015). Other work has shown that the ratio of [C II to TIR]
luminosity exhibits a strongly nonlinear but quantifiable trend with SFR surface density(Smith et al.2017).
In order to further decode any relationship between [C II]158μm emission and SFR, the [C II emission can be] separated by the ISM phase in which it originates. The [N II]205μm line is a powerful tool for separating [C II] emission into neutral and ionized ISM components. As the [N II]205μm line is also a far-infrared fine-structure line, it is typically unattenuated by dust, similar to the [C II]158μm line. Since neutral nitrogen has an ionization energy of 14.5eV, slightly above that of hydrogen at 13.6eV, N+ mainly exists in environments where hydrogen is ionized; [N II]205μm emission is essentially from H II regions and other ionized phases of the ISM only. Due to this constraint on the origin of the[N II]205μm emission, [N II can be used to] constrain the fraction of[C II emission arising from the ionized] ISM. Since the[N II]205μm line has a similar critical density to the [C II]158μm line (∼32 cm−3 for [N II 205] μm and ∼45 cm−3 for [C II 158] μm, Oberst et al.2006; Croxall et al.
2017),the ratio of [C II]158μm emission to [N II]205μm emission is nearly constant regardless of electron number density(see Figure2, and Croxall et al.2012). This consistency
makes [N II]205μm derived measures of the fraction of [C II]158μm emission from the ionized ISM less dependent on electron number density(ne) than when using the brighter
[N II]122μm line, because the ratio of the [C II]158μm and [N II]122μm lines varies by a factor of three in conditions of normal ISM density (Croxall et al. 2012). Therefore,
[N II]205μm emission is the preferred tool to distinguish between the [C II originating from the ionized and neutral] phases of the ISM.
With the added information from the[N II]205μm line, the [C II]158μm emission can be separated by ISM phase. Isolated [C II emission from the ionized and neutral ISM phases can] then be calibrated and tested as indicators of SFR. Unfortunately, due to the weakness of the[N II]205μm line and its location in the far-infrared part of the spectrum, it is notoriously difficult to detect in galaxies in the local universe. Fortunately, a collection of[N II]205μm detections was made with the Herschel Space Observatory(Pilbratt et al.2010; Poglitsch et al.2010).
An intriguing application of an improved[C II -based SFR] indicator lies in the realm of high-redshift galaxies. There have been many detections of the[C II]158μm line in galaxies from z∼1 to z∼7 (Hailey-Dunsheath et al.2010; Ivison et al.2010; Stacey et al. 2010; Graciá-Carpio et al. 2011; Valtchanov et al.
2011; Gullberg et al. 2015, 2018; Barisic et al. 2017; Malhotra et al.2017; Rybak et al. 2019). The brightness of the [C II line] enables detections across a wide range of distances, making it a popular tool for probing the PDR properties and kinematics of galaxies in the high-redshift universe. In addition to the availability of high-redshift[C II detections, the Atacama Large Millimeter/] submillimeter Array has already detected [N II] in multiple galaxies beyond a redshift of four(Decarli et al. 2014; Aravena et al. 2016; Pavesi et al.2016; Lu et al. 2017). Other work has
identified the [C II and [] N II lines in larger samples of galaxies] with 1<z<2 (Stacey et al.2010), where cosmic star formation
peaks(Madau & Dickinson2014). As the [C II]158μm emission line is now measurable both in the local universe and in distant galaxies, it can be used to trace star formation at nearly any cosmic epoch. Also, unlike ultraviolet and optical star formation tracers such as FUV and Hα, attenuation by dust is typically negligible at 158μm.
This paper uses[C II]158μm and [N II]205μm measure-ments from the Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH) (Kennicutt et al.
2011) and Beyond the Peak (BtP) (Pellegrini et al.2013) data
sets to decompose the [C II]158μm emission into the ISM phases in which it originates. These samples include normal galaxies in the local universe(D30 Mpc). This work builds on the previous studies of these data sets in Herrera-Camus et al.(2015), Croxall et al. (2017), and Abdullah et al. (2017).
In Section 2, the properties of the nuclear and extranuclear star-forming regions investigated and the observations used in this work are described. Section3explains the processing done to evaluate the data. In Section 4we present the results of our analysis, comparing the measurements of [C II from isolated] ISM phases to TIR luminosity, emission from small dust grains measured by the strength of emission features of polycyclic aromatic hydrocarbons (PAHs), and SFR. Section 5 provides the conclusions drawn from this work.
2. Sample and Observations 2.1. KINGFISH Galaxies
Table 1 provides a brief description of the properties of the galaxies included in this study. This work uses the subset of the galaxies in the KINGFISH sample with Photoconductor
Array Camera and Spectrometer (PACS) [N II]205μm and [C II]158μm spectral maps (Kennicutt et al.2011). The overall
KINGFISH survey studied 61 galaxies in the local universe (D<30 Mpc) spanning a wide range of morphological types, luminosities, metallicities, and levels of star formation activity. Of these 61 galaxies, 54 were observed at the [C II]158μm line. Within this smaller sample, Herschel PACS far-infrared spectral cubes targeting the[N II]205μm line were acquired for galaxies with the highest far-infrared surface brightnesses. This subsample contains 31 regions in 28 galaxies, with 24 centered on the brightest, central nuclear region of the galaxy and seven centered on extranuclear star-forming regions in the disk of the galaxy. All of the targeted galaxies in this sample are normal star-forming galaxies in terms of infrared luminosity, with no LIRGs included, i.e., all KINGFISH galaxies have log10(LIR L)11. The galaxies included in this work cover a range of nebular metallicities (12 + log(O/H)) spanning ∼8.1–8.7 as measured by Moustakas et al. (2010) using the calibration of Pilyugin &
Thuan (2005), and a distance range of 3–30Mpc (Kennicutt
et al.2011). The physical scales of the [C II]158μm∼11″ and the[N II]205μm∼14 5 point-spread functions (PSFs) lie in the ranges 180–1700pc and 200–2100pc, respectively, across the sample. The proximity and properties of the galaxies included in this sample allow us to determine the nature of the[C II emission] in galaxies with a wide array of spectral coverage and compare
Table 1 Sample
Galaxy Alternative Region R.A. Decl. Distance LTIR Morph. Type BtP
Name Type (J2000) (J2000) (Mpc) (Le) Data
NGC0337 Nuclear 00:59:50.200 −07:34:38.00 19.3 1.2×1010 SBd ...
NGC0628 [H76]292 Extranuclear 01:36:45.200 +15:47:49.00 7.20 8.0×109 SAc ...
NGC1097 UGCA 041 Nuclear 02:46:19.200 −30:16:28.00 14.2 4.5×1010 SBb ✓
NGC1266 Nuclear 03:16:00.600 −02:25:39.00 30.6 2.5×1010 SB0 ✓
NGC1377 Nuclear 03:36:39.500 −20:54:08.00 24.6 1.3×1010 S0 ...
IC 342 UGC 02847 Nuclear 03:46:48.200 +68:05:48.00 3.28 1.4×1010 SABcd ...
NGC1482 Nuclear 03:54:38.900 −20:30:09.00 22.6 4.4×1010 SA0 ✓
NGC2146 UGC 03429 Nuclear 06:18:38.100 +78:21:23.00 17.2 1.0×1011 Sbab ...
NGC2798 UGC 04905 Nuclear 09:17:22.800 +42:00:00.00 25.8 3.6×1010 SBa ✓
NGC2976 [HK83]58 Extranuclear 09:47:07.300 +67:55:56.00 3.55 9.0×108 SAc ✓
NGC3049 UGC 05325 Nuclear 09:54:49.600 +09:16:18.00 19.2 3.5×109 SBab ...
NGC3077 UGC 05398 Nuclear 10:03:18.900 +68:44:03.00 3.83 6.4×108 I0pec ✓
NGC3351 M095 Nuclear 10:43:57.900 +11:42:13.00 9.33 8.1×109 SBb ✓
NGC3521 UGC 06150 Nuclear 11:05:48.600 −00:02:05.00 11.20 3.5×1010 SABbc ✓
NGC3627 MJV 14274 Extranuclear 11:20:16.500 +12:58:42.00 9.38 2.8×1010 SABb ✓
NGC4254 M099 Nuclear 12:18:49.500 +14:24:55.00 14.4 3.9×1010 SAc ✓
NGC4321 M100 Nuclear 12:22:54.900 +15:49:19.00 14.3 3.5×1010 SABbc ✓
NGC4536 UGC 07732 Nuclear 12:34:27.000 +02:11:19.00 14.5 2.1×1010 SABbc ✓
NGC4569 M090 Nuclear 12:36:49.800 +13:09:47.00 9.86 5.2×109 SABab ✓ NGC4631 UGC 07865 Nuclear 12:42:07.700 +32:32:35.00 7.62 2.4×1010 SBd ✓ NGC4736 [HK83]004 Extranuclear 12:50:56.500 +41:07:09.00 4.66 5.8×109 SAab ✓ NGC4826 M064 Nuclear 12:56:43.500 +21:41:03.00 5.27 4.2×109 SAab ✓ NGC5055 M063 Nuclear 13:15:49.000 +42:01:44.00 7.94 2.2×1010 SAbc ✓ NGC5457 [HK83]033 Extranuclear 14:03:41.300 +54:19:03.00 6.70 2.3×1010 SABcd ...
NGC5457 UGC 09013 Nuclear 14:03:12.800 +54:20:52.00 6.70 2.3×1010 SABcd ...
NGC5713 UGC 09451 Nuclear 14:40:11.400 −00:17:22.00 21.4 3.2×1010 SABbcp ✓
NGC5866 UGC 09723 Nuclear 15:06:29.500 +55:45:44.00 15.3 5.7×109 S0 ...
NGC6946 UGC 11597 Nuclear 20:34:52.300 +60:09:13.00 6.80 8.6×1010 SABcd ✓
NGC6946 [HK83]003 Extranuclear 20:35:25.400 +60:10:00.00 6.80 8.6×1010 SABcd ✓
NGC6946 [HK83]066 Extranuclear 20:35:11.400 +60:08:59.00 6.80 8.6×1010 SABcd ✓
NGC7331 UGC 12113 Nuclear 22:37:04.000 +34:24:53.00 14.5 5.3×1010 SAb ✓
Note.Sample selected based on availability of KINGFISH PACS [C II]158μm and [N II]205μm spectral maps. Measurements of distance, LTIR, and morphological
our results to the U/LIRGS studied in Díaz-Santos et al. (2017).
Further, by only targeting normal, star-forming galaxies we are able to explore the behavior of the [C II deficit without the] complicating effects of AGNs or other extreme conditions. Establishing an understanding of the processes occurring in these well-studied galaxies will lay the groundwork for understanding measurements of more extreme cases.
In addition to the KINGFISH measurements, 20 of the 28 galaxies in this sample were included in the BtP survey. The BtP survey used the Spectral and Photometric Imaging Receiver (SPIRE) on Herschel to obtain [N II]205μm maps with a larger area, extending this study to the more quiescent areas surrounding the bright regions of star formation included in the KINGFISH PACS 205μm survey. The larger BtP maps introduce 127 additional 20″ regions within these 20 galaxies. For more information about the choice of 20″ regions, see Section3. As the BtP regions cover a wider range of conditions, we split them into those centered nearest the galaxy nuclei and those further removed from the nuclei. We distinguish these “Inner” and “Outer” regions as those within one-quarter of R25
and those outside of one-quarter of R25. The numbers of each
type of detection are listed in Table1. Although the BtP regions do cover a wider range of properties, the centers of only∼14% fall outside of one-quarter of R25.
2.2. KINGFISH PACS Line Maps
This work uses the KINGFISH program’s Herschel/PACS far-infrared mapped spectral observations of the 158 and 205μm lines. All PACS spectral maps were obtained in the unchopped mapping mode and reduced using the Herschel Interactive Processing Environment (HIPE) version 11.2637 (Smith et al.2017). Standard data reductions were applied to all
images and are summarized in Croxall et al. (2012). The
resulting line maps cover a 47″ by 47″ square field of view with 2 6 pixels in both 158 and 205 μm lines and have calibration uncertainties of 20% and 30%, respectively(Beirão et al.2010; Croxall et al.2012). The detection of the [N II]205μm line in nearby galaxies at a spectral resolution of 150 km s−1is unique to the PACS instrument, making this data set invaluable for understanding the far-infrared fine-structure lines in the nearby
universe (Beirão et al. 2010). For two of the galaxies in this
sample, NGC5457 and NGC6946, multiple star-forming regions were targeted; all others were observed only at the central nuclear region or at a single extranuclear star-forming region. As an example of the data from a typical nuclear pointing in this sample, the[C II]158μm and [N II]205μm maps of IC342 can be seen in Figure1. Theflux measurements from the PACS line maps are shown in Table2.
2.3. Beyond the Peak SPIRE-FTS Line Maps
In addition to the KINGFISH PACS[N II]205μm observa-tions, the SPIRE-FTS on Herschel mapped the[N II]205μm line in 20 of the KINGFISH galaxies with PACS[N II]205μm detections as part of the Beyond the Peak program. These observations were obtained using the SPIRE-FTS intermediate mapping mode, which is a four-point dither(Croxall et al.2017).
These maps were calibrated using the extended-flux HIPEv10 package. The spectral resolution for these data is∼300 km s−1at the 205μm wavelength. More information about the observa-tions and data processing can be found in Pellegrini et al.(2013)
and A. Crocker et al. (2019, in preparation). The SPIRE-FTS beam size at 205μm is 16 6 (Makiwa et al.2013), and the maps
produced by this survey are significantly larger than the PACS [N II]205μm maps from the KINGFISH survey (2′ × 2′ versus 1′ × 1′ in most cases). The inclusion of these larger spectral maps extends the range of ISM conditions covered to cooler quiescent material surrounding the star-forming regions included in the KINGFISH sample. Some of the important properties of these maps are listed in Table3.
2.4. PACS Imaging
As part of the KINGFISH survey, PACS images at 70, 100, and 160μm were obtained in scan mode for each galaxy in this sample. These data provide valuable information about the dust temperatures of our sample and were used to determine the far-infrared colors and total far-infrared luminosities for our regions. These measurements were then used to quantify the [C II] deficit for our sample (see Section 4.1). Uniform surface
brightness sensitivities of σsky∼5, 5, and 2MJysr−1 at
70μm, 100μm, and 160μm respectively, were achieved for
Figure 1. HerschelPACS [C II]158μm (left) and [N II]205μm (right) maps of IC342 with the 20″ region where fluxes were extracted marked as black circles. Intensities are measured in nW m−2sr−1. The nuclear region of IC342 was selected as a typical region in this study, because the measured fluxes in both [C II and] [N II are reliably detected.]
each region(Dale et al. 2012). The KINGFISH PACS images
have a calibration uncertainty of cal,n nf ~5%. For more information about these far-infrared images, see Dale et al. (2012). A summary of the details of these images can be found
in Table4.
2.5. Ancillary Infrared Observations
Most regions in the KINGFISH sample were also included in the Spitzer Infrared Nearby Galaxies Survey(SINGS) (Kennicutt et al. 2003; Dale et al. 2006; Smith et al. 2007b). This
survey used the Spitzer Space Telescope to obtain infrared imaging and spectroscopy for 75 nearby galaxies. The Infrared
Spectrograph (IRS) on board Spitzer obtained low-resolution (R∼50–100) spectral maps in both the short–low (SL) (5–14 μm) and long–low (LL) (14–38 μm) modules for most of the nuclear regions in the KINGFISH sample (Smith et al.
2007b). This wavelength range includes several prominent
PAH emission features. The availability of these data allows comparisons between the PAH emission and the[C II emission] for the regions in our sample(see Section4.2). The extranuclear
regions targeted by the KINGFISH survey were observed by Spitzer/IRS in only the SL module. In most cases, the larger BtP maps were covered with Spitzer/IRS by only the LL module. All observations were reduced with the CUBISM program to create spatially resolved spectral cubes and processed using the IRS pipeline version S14, producing an absoluteflux calibration uncertainty of∼25% (Dale et al.2006). For more information on
these observations and data processing, see Smith et al.(2007a).
As part of the SINGS survey, 3.6, 8.0, and 24μm images were also obtained for each galaxy using the Infrared Array Camera(IRAC) and the Multi-band Imaging Photometer (MIPS) (Rieke et al.2004). These images were then processed using the
MIPS Instrument Team Data Analysis Tool (Gordon et al.
2005). Calibration errors for these images are 10% at all three
wavelengths(Dale et al.2005). Four of the galaxies in this work,
IC342, NGC2146, NGC3077, and NGC5457, were not included in the SINGS sample. For these galaxies 3.6, 8.0, and 24μm images were obtained from other archival Spitzer surveys, namely the Local Volume Legacy (LVL, Dale et al.
2009) and MIPS GTO (Pahre et al.2004). The 3.6 and 8.0μm
images provide alternative measurements of PAH feature strength for regions without both SL and LL coverage. For the purposes of this study, the 24μm data were incorporated into the determination of the total infrared luminosities as well as in our measurements of SFR. More information about these images can be found in Table4.
2.6. Ancillary Ultraviolet Data
In order to determine SFR, FUV maps were obtained from GALEX. 26 of the 31 regions in this sample were imaged as part of the GALEX Nearby Galaxy Survey(NGS) (Gil de Paz et al. 2005), and those that were not covered by NGS were
imaged as part of the GALEX All-Sky Imaging Survey(AIS) or by other programs, with the exception of NGC1377 which has no GALEX imaging. Priority was given to long-exposure data when available. Exposure times for this sample range from 110 to 21,177 s, with a median exposure time of ∼1700 s. The GALEX images have a diffraction-limited FWHM of∼6″ (Gil de Paz et al.2005). For more details on the GALEX images, see
Table4.
Table 2 PACS Line Measurements
Galaxy Region L([C II 158] μm) L([N II 205] μm)
Name Type (erg s−1) (erg s−1)
NGC0337 Nuclear 774(±77)×1038 <6.44×1038 NGC0628 Extranuclear 491(±49)×1037 52.3(±16)×1037 NGC1097 Nuclear 277(±28)×1039 165(±50)×1038 NGC1266 Nuclear 116(±12)×1039 108(±33)×1038 NGC1377 Nuclear 142(±15)×1038 97.0(±29)×1038 IC 342 Nuclear 196(±20)×1038 178(±53)×1037 NGC1482 Nuclear 759(±76)×1039 275(±83)×1038 NGC2146 Nuclear 178(±18)×1040 48.0(±14)×1039 NGC2798 Nuclear 406(±41)×1039 236(±71)×1038 NGC2976 Extranuclear 425(±43)×1037 106(±32)×1036 NGC3049 Nuclear 604(±60)×1038 24.4(±7.7)×1038 NGC3077 Nuclear 759(±76)×1038 159(±48)×1036 NGC3351 Nuclear 443(±44)×1038 33.8(±10)×1038 NGC3521 Nuclear 395(±40)×1038 274(±83)×1037 NGC3627 Extranuclear 765(±77)×1038 52.5(±16)×1038 NGC4254 Nuclear 103(±10)×1039 84.5(±25)×1038 NGC4321 Nuclear 107(±11)×1039 125(±38)×1038 NGC4536 Nuclear 300(±30)×1039 39.5(±12)×1038 NGC4569 Nuclear 198(±20)×1038 <1.80×1038 NGC4631 Nuclear 670(±67)×1038 223(±67)×1037 NGC4736 Extranuclear 104(±10)×1038 27.7(±8.4)×1037 NGC4826 Nuclear 179(±18)×1038 151(±45)×1037 NGC5055 Nuclear 227(±23)×1038 227(±68)×1037 NGC5457 Extranuclear 194(±19)×1038 5.29(±2.9)×1037 NGC5457 Nuclear 530(±53)×1037 101(±30)×1037 NGC5713 Nuclear 349(±35)×1039 151(±45)×1038 NGC5866 Nuclear 224(±22)×1038 43.5(±13)×1038 NGC6946 Nuclear 606(±61)×1038 302(±91)×1037 NGC6946 Extranuclear 216(±22)×1038 109(±33)×1037 NGC6946 Extranuclear 965(±97)×1037 157(±47)×1037 NGC7331 Nuclear 760(±76)×1038 91.9(±28)×1038
Note.Luminosities are measured for regions of 20″ radius smoothed to a 20″ FWHM PSF(see Section3for more information). 10% and 30% calibration
uncertainties are included for [C II]158μm and [N II]205μm luminosities respectively.
Table 3
Summary of Spectral Map Data
Telescope Instrument λ ∼PSF FWHM (μm) (arcsec) Spitzer IRS SL 5–14 2–3 Spitzer IRS LL 14–38 3–10 Herschel PACS 158 11.4 Herschel PACS 205 14.5 Herschel SPIRE 205 14.5 Table 4 Summary of Imaging Data
Telescope Band Pixel Scale ∼PSF FWHM
(arcsec) (arcsec) GALEX FUV 1.5 6.0 Spitzer IRAC 3.6μm 1.2 1.7 Spitzer IRAC 8.0μm 1.2 2.0 Spitzer MIPS 24μm 1.5 6.0 Herschel PACS 70μm 1.40 5.6 Herschel PACS 100μm 1.70 6.7 Herschel PACS 160μm 2.85 11.4
3. Data Analysis
To support a consistent analysis across the wide range of wavelength information we use (0.15–205 μm), we have extracted all fluxes according to an effective 20″ resolution. This consistency in extraction is accomplished by taking each image (and each spectral slice within the IRS cubes) at its native resolution and, after centering on each targeted region, summing the pixel values using a Gaussian weightingσextraction
determined by the difference in quadrature between the desired 20″ and native resolutions, namely
( )
sextraction2 =s202 -snative2 . 1 In this procedure the data are left at the native pixel sampling. This allows for streamlined convolution to an equivalent Gaussian resolution profile across the wide range of wave-length images we are working with. This method can be consistently used with both images and cubes, ensuring that all data are processed in the same manner. This process follows a similar method to that used in Contursi et al. (2002), which
found little difference between this method and other methods of smoothedflux extractions.
3.1. Inner and Outer BtP Measurements
As multiple studies have found the[C II deficit to be a local] effect (Smith et al. 2017; Gullberg et al. 2018), we use the
extended SPIRE coverage to determine whether there are significant differences between the behavior of the phase-separated [C II deficit in the inner nuclear regions of these] galaxies and in the more extended disks. In order to test these differences, we divide the BtP regions into“Inner” and “Outer,” with Inner regions centered within 0.25R25and Outer regions as
any falling outside this limit. As the [N II]205μm emission is faint, only ∼14% of the BtP regions with [N II]205μm detections fall into our definition of Outer regions. To avoid biasing our data toward the conditions within galaxies with more regions, an average of the SPIRE detections for the Inner and Outer regions of each galaxy included in the BtP survey was performed to produce the BtP Inner and Outer measurements used throughout this study.
3.2. Neutral Fraction of[CII] Measurements
To determine the fraction of the[C II emission originating in] the neutral phase of the ISM, the following relation from Croxall et al. (2012) and Oberst et al. (2006) was used:
[ ] ( [ ] ) [ ] ( ) [ ] = - ´ f C II 158 R N II 205 C II 158 . 2 C II ,Neutral Ionized
In this equation, RIonized is the expected ratio of
[C II]158μm to [N II]205μm emission in the ionized gas where both C+and N+are present, as represented by the solid cyan line in Figure 2. This ratio is derived using the collision rates of Tayal(2008) for [C II and Tayal] (2011) for [N II and] assuming Galactic gas-phase abundances of carbon and nitrogen (1.6×10−4per hydrogen atom and 7.5×10−5 per hydrogen
atom respectively) (see Croxall et al. 2017 for further
information). It has been found that there is a slight dependence of RIonized on gas-phase abundances, but this dependence only
results in shifts of5% on measurements of f[C II ,Neutral] across the range of metallicities included in this sample, and therefore does not affect our results (Croxall et al. 2017). RIonized is
determined for each region individually based on measurements of electron number density made using ratios of the[SIII]18.7 and 33.4μm lines (Dale et al. 2006) or the [N II]122 and 205μm lines (Herrera-Camus et al. 2016). Over the range of
conditions covered by our sample, these two ratios both provide reliable measurements of ne (Rubin et al. 1995). Any slight
differences in calculations of nebetween these two methods will
not affect our results,because RIonizedis nearly independent of
ne. Studies of local galaxies with high specific SFR (sSFR) have
found that there is a correlation between neand sSFR that could
potentially affect the ionization parameter in the most dense regions in our study (Kewley et al. 2015; Bian et al. 2016; Holden et al.2016), but we have not modified our calculation of
RIonizedto account for this.
The values of f[C II ,Neutral] for the KINGFISH and BtP samples are shown in Figure3, plotted againstnf 70 mn( m )/ (nf 160 m ,n m ) a proxy for dust temperatures. Also shown in Figure3 are the values determined using smaller subregions measured using Herschel SPIRE data in Croxall et al. (2017). The f[C II ,Neutral]
values found for this sample follow a similar trend to those in the study of Croxall et al.(2017), with an average neutral fraction of
∼67% and a decreasing dynamic range in f[C II ,Neutral] for the more actively star-forming galaxies that have increased
( )
nf 70 mn m / (nf 160 mn m ) values. In this and all subsequent figures, the KINGFISH regions, shown as magenta squares, cover only highernf 70 mn( m )/ (nf 160 m ratios because alln m ) KINGFISH regions are centered on warmer star-forming regions, while the BtP data with a wider field of view extend to the quiescent environments surrounding these star-forming regions.
3.3. PAH Measurements
The PAHFIT program was used to determine the strength of the PAH emission features from the fluxes extracted from
Figure 2.Ratios of [C II]158μm to [N II]122μm and 205μm emission for ionized regions where both C+and N+are present. Ratios are determined using the method described in Croxall et al.(2017).
the IRAC spectral maps (Smith et al.2007b; Gallimore et al.
2010). This program separates emission from PAH features,
far-infrared emission lines, warm dust, and stars. For the KINGFISH nuclear regions with IRAC spectral maps in both SL and LL large enough to cover the [C II]158μm and [N II]205μm emission, the total PAH power was determined by summing the emission from all the PAH emission features measured by PAHFIT (Croxall et al. 2012). For regions
covered only in SL (KINGFISH extranuclear regions) or only in LL(BtP regions), the total PAH power was determined using a combination of the IRAC spectral maps and the 8.0 and 3.6μm IRAC imaging. The bandpass used for the 8.0μm images contains one of the strongest PAH features (Croxall et al. 2012). Stellar contributions can be removed from the
8.0μm flux through the use of the 3.6μm IRAC images. The total PAH power can thus be calculated photometrically using the result determined from the LVL survey (Marble et al.
2010): ( ) ( ) ( ) n n = n - ´ n * S S PAHPhot 8.0 0.24 3.6 . 3
With the added information from either the SL or LL bands, this photometrically determined PAH power can be improved using the results from Croxall et al.(2012):
[ n ( )] ( )
= ´ + ´ Sn
PAHSL* 0.497 PAH*Phot 3.59 11.3 4
[ n ( )] ( )
= ´ + ´ Sn
PAH*LL 0.7472 PAH*Phot 3.25 17 . 5
With the Spitzer IRAC imaging and spectral maps, estimates of the total PAH emission for most regions in our sample were determined, allowing for comparisons of the PAH emission and [C II emission in both actively star-forming regions and the] more quiescent environments surrounding these regions. For our sample, the total PAH emission strength of the KINGFISH extranuclear regions is determined using Equation(4) and that
of the BtP regions is determined with Equation(5). These three
methods produce similar measurements of total PAH emission power with 1σ scatters of 0.15dex and 0.09dex compared to the summation of all PAH features for Equations (5) and (4)
respectively (Croxall et al.2012).
4. Results and Discussion 4.1.[NII] and [CII] Deficits
The[N II]/TIR measurements for our sample are displayed as a function of the far-infrared color as measured by the
( ) ( )
nfn 70 mm nfn 160 m ratio in Figurem 4. This ratio is used because it is a proxy for dust temperature, and increased dust temperatures often indicate increased star formation activity. TIR luminosity was determined for each individual region using Equation(4) from Dale et al. (2014) (Equation (6) in this
work) with the Spitzer 24μm luminosities and Herschel PACS 70 and 160μm luminosities: ( ) ( ) ( ) ( ) n m n m n m = + + n n n L L L L 1.548 24 m 0.767 70 m 1.285 160 m . 6 TIR
Similar to thefindings of Díaz-Santos et al. (2017), we find that
the[N II]205μm line ratio with TIR shows a clear decreasing trend in warmer regions, and this trend holds irrespective of whether the inner or outer portions of the galaxies are sampled. Using the form
[ ] ( ) ( ) ( ) n m n m =m fn fn +b log N II TIR 70 m 160 m 7 10
we perform a linear regression for all the individual regions in our sample, the averaged inner BtP measurements and the KINGFISH nuclear regions, and the averaged outer BtP measurements and the KINGFISH extranuclear regions. For these and all followingfits, the KINGFISH extranuclear region from NGC5457 has been ignored because it is a faint source with significant noise contamination at the edges of the image, making it an outlier in each plot (see the cyan diamond with
Figure 4.[N II]205μm/TIR plotted against the far-infrared color measured bynfn(70 mm ) nfn(160 m . The decreasing trend as a function of warmerm ) regions shows that the [N II]205μm line has a deficit in our sample. Lines of bestfit are shown for the trend found with all the individual regions in the KINGFISH and BtP samples(black line), the KINGFISH nuclear regions and the averaged BtP inner regions (orange dashed line), and the KINGFISH extranuclear regions and the averaged BtP outer regions(blue dotted line). Figure 3. f[C II ,Neutral] measurements for the nuclear regions in the KINGFISH
sample(magenta squares), extranuclear regions in the KINGFISH sample (cyan diamonds), averaged inner regions in the BtP sample (orange stars), averaged outer regions in the BtP sample(blue plus marks), and all individual BtP regions (green crosses), plotted along with the data from Croxall et al. (2017), shown as
gray points. Both samples show a similar pattern of increased neutral fraction with far-infrared color as measured by thenf 70 mn( m )/ (nf 160 m ratio.n m )
[ ]
log10 N IITIR just below −3.0 in Figure 4). The slopes and
intercepts for each fit are listed in Table 5 and displayed in Figure4. Eachfit has an equivalent slope within error, showing that the location of the region within the galaxy does not significantly affect our results.
In Figure 5, the[C II]/TIR measurements are displayed as a function ofnfn(70 mm ) nfn(160 m . In thism ) figure, the left panel is the combined ionized and neutral [C II emission] ([C II] I N+ ), the middle panel is the [C II emission from only the ionized ISM] ([C II]Ionized), and the right panel is the [C II emission from only]
the neutral ISM ([C II]Neutral). The trend we notice in our [C II] Ionized measurements is a scaled version of the [N II]205μm
measurements by nature of our method for determining [C II]
Ionized. Each[C II]/TIR measure is fit by a linear regression of
the form [ ] ( ) ( ) ( ) n m n m =m fn fn +b log C II TIR 70 m 160 m . 8 10
These fits are displayed in Figure 5 and the properties of each fit are listed in Table 6. For our sample, we find an average oflog10[ ]+ = -2.53 C II TIR I N , [ ] = -log10 3.07 C II TIR Ionized , and [ ] = -log10 2.73 C II TIR
Neutral . These results match well with other studies, which find the [C II line emission to account for] approximately 1% of the total infrared emission (Smith et al.
2017). As expected from previous work (Croxall et al. 2012; Smith et al. 2017), the ratio of combined ionized and neutral
[C II luminosity to TIR luminosity shows a decline with] warmer far-infrared color. As there are no extreme cases in this study, the decreasing trend in the combined ionized and neutral [C II]/TIR ratio is slight, as shown by the slope of −0.127 for our line of bestfit. While the [C II deficit is pronounced for the] [C II emission from the ionized ISM, it disappears when only] [C II emission from the neutral ISM is considered, as shown in] the middle and right panels of Figure 5.
By separating our detections by ISM phase, we are able to narrow down the possible causes of the [C II deficit in our] sample. Using this method, we determine that the decreasing trend in [C II]/TIR for warmer, more actively star-forming environments is greatly reduced when only the [C II emission] from the neutral phases of the ISM is considered. On the other hand, the ratio of[C II emission from the ionized phases of the] ISM to TIR luminosity shows a steep decrease as a function of
far-infrared color. This can also be seen in the combined ionized and neutral[C II]/TIR measurements, where the slight decrease observed is driven by the regions from the BtP survey, which have lower f[CII ,Neutral] values and therefore more emission from the ionized phases of the ISM (see Figure 5).
This trend suggests that the cause of the [C II de] ficit occurs predominantly in the ionized phases of the ISM, a conclusion that is supported by the work done in the GOALS survey (Díaz-Santos et al.2017).
This decreasing trend in [C II from the ionized phases of the] ISM holds both for the star-forming regions targeted in the KINGFISH study, shown as magenta squares, and for the more extended coverage from the BtP survey, shown as green crosses, and is true for both regions within 0.25R25and those outside this
boundary, as seen in Figure 5 and in the consistency of our measured slopes within error for each location. The decreasing trend measured for the [C II]I N+ /TIR luminosity ratio is much shallower than the steep trend found in the[N II]205μm and [C II]Ionized measurements, shown in Figure 4. This difference
indicates that the lack of a measured deficit in the [C II emission] from the neutral ISM is not solely caused by identical decreases in the[C II]158μm and [N II]205μm fluxes. If this were the case we would then expect similar slopes in our linefits to the [C II]I+N and [N II] deficits. Instead, there must be some
differing physical processes in the neutral and ionized ISM driving the observed deficit.
With this new insight into the nature of the[C II deficit, we] can narrow down the possible causes of the deficit in our sample. We suggest that the[C II deficit is caused by changes] in the fraction of UV light absorbed by dust within H II regions and PDRs. Compact/nuclear regions with warmer far-infrared colors have fractionally higher UV absorption by dust.
( ) ( )
nfn 70 mm nfn 160 m traces TIR/FUV for centrally con-m
centrated distributions of dust (Dale et al. 2007); therefore
higher nfn(70 mm ) nfn(160 m indicates regions where UVm ) light is being proportionally more quenched by dust in PDRs and H II regions. The resulting dearth of UV emission leaking into the diffuse ISM leads to a decrease in[C II emission from] the ionized phases of the ISM. Previous studies have suggested that a majority of the [C II]Ionizedemission must originate in the
diffuse ISM because there should be little [C II emission] originating in H II regions where the [C II] line is often thermally quenched by high temperature and densities, and the availability of photons with energies above the 24.38 eV necessary to ionize C+ limits the emission of the 158μm line (Díaz-Santos et al.2017; Herrera-Camus et al. 2018a). Thus,
the decrease in [C II]Ionized as a fraction of the total infrared
emission may be due to(1) a smaller fraction of the ultraviolet radiation from stars being above the Lyman limit (i.e., a deficiency in recent star formation) and (2) a smaller fraction of the H-ionizing radiation being absorbed in low-density (ne<30 cm−3) H II where [C II emission is not collisionally]
quenched, i.e., the diffuse ionized ISM. Under these conditions, [C II]Neutral should remain unaffected because it primarily
originates in PDRs, where UV photons with energies above 11.3 eV have not been significantly quenched and are still available to ionize the carbon atoms present. This interpretation is consistent with the explanations of the[C II deficit described] in Abel et al. (2009) and Graciá-Carpio et al. (2011) and
suggests that the third and fourth explanations of the deficit listed in Section 1 (i.e., the thermalization of [C II in H II] regions and increased FUV absorption leading to [O I]
Table 5 Linear Fits from Figure4
m b Rms Scatter
All Individual Regions
−0.381 [±0.03] −3.05 0.174
All Inner Regions
−0.355 [±0.04] −3.09 0.181
All Outer Regions
−0.317 [±0.22] −3.08 0.246
Note.Properties of the lines of bestfit for our [N II de] ficit measurements, displayed in Figure 4. We divide ourfits by region type, with a fit for all individual regions, afit for the averaged BtP inner regions and the KINGFISH nuclear regions, and afit for the averaged BtP outer regions and the KINGFISH extranuclear regions.
becoming a major coolant in the diffuse ionized ISM) are the most likely for our sample. It is also possible that the other mechanisms described in Section1have an effect on the deficit measurements for our sample, but do not explain the changes in trends between measurements of the ionized-phase [C II and] the neutral-phase[C II .]
4.2. [CII]158μm and PAH Emission
PAHs are the dominant source of the photoejected electrons that heat the neutral ISM, which is in turn cooled through channels such as the[C II]158μm line (Bakes & Tielens1994;
Weingartner & Draine2001). The mid-infrared PAH emission
features are a result of vibrational and bending transitions that have been excited by the absorption of far-ultraviolet photons. We thus compare the strength of the PAH emission features to the[C II emission from both the neutral and ionized phases of] the ISM to better understand the microphysics underlying the gas heating by photoelectric ejection of electrons from PAHs and gas cooling by [C II]158μm emission. Previous works have found that while the [C II]158μm/TIR decreases in warmer regions, the ratio of [C II]/PAH emission is more constant(Helou et al.2001; Croxall et al.2012). By measuring
this ratio in the separated ISM phases we can test the relationship between gas heating by small grains and gas cooling by[C II emission.]
The ratio of[C II luminosity to the strength of PAH features] for the combined ionized and neutral [C II emission and the] [C II emission from only neutral and ionized phases of the ISM] can be found in Figure 6. KINGFISH nuclear regions where both SL and LL IRS cubes are available are shown as magenta squares, KINGFISH extranuclear regions where only SL IRS cubes are available are shown as blue squares, and BtP regions where only LL IRS cubes are available are shown as green crosses. It should be noted that the slight separation between the different region types is likely driven by the different methods for determining PAH emission strengths, and not by any differences between the regions themselves (see Section3.3for more information).
The middle and right panels of Figure6 show the [C II to] PAH emission ratio when only the [C II emission from the] ionized and neutral ISM are considered, respectively. Similar to results of the deficit observed when comparing the [C II and] TIR luminosity, the ratio of [C II emission from only the] ionized ISM to PAH emission feature strength shows a clear decrease as a function of far-infrared color, while the ratio of [C II emission from only the neutral ISM to PAH emission] feature strength remains fairly constant across the range of
Figure 5.Left: [C II]158μm/TIR plotted against the far-infrared color measured bynfn(70 mm ) nfn(160 m . The [m ) C II de] ficit is observed as a slight decrease in the [C II]158μm/TIR ratio at warmer far-infrared colors in our sample. Our sample covers a limited range of conditions, and therefore only a small deficit effect is observed. The lines represent predicted neutral fractions based on the relationship determined by the [C II emission from only the ionized phase of the ISM] (shown in the middle panel). Middle: the ratio of [C II]158μm emission from the ionized ISM to TIR plotted against the far-infrared color measured by
( ) ( )
nfn 70 mm nfn 160 m . Right: the ratio of [m C II]158μm emission from the neutral ISM to TIR plotted against the far-infrared color measured by
( ) ( )
nfn 70 mm nfn 160 m . Notice the disappearance of the observed decrease in [m C II]158μm/TIR when only the neutral ISM is considered.
Table 6 Linear Fits from Figure5
[C II Component] m b Rms Scatter
All Individual Regions
Ionized+ Neutral −0.127 [±0.03] −2.33 0.175
Ionized −0.360 [±0.03] −2.49 0.173
Neutral −0.024 [±0.03] −2.69 0.270
All Inner Regions
Ionized+ Neutral −0.113 [±0.05] −2.36 0.197
Ionized −0.356 [±0.04] −2.49 0.171
Neutral −0.024 [±0.06] −2.69 0.287
All Outer Regions
Ionized+ Neutral −0.050 [±0.08] −2.30 0.083
Ionized −0.304 [±0.26] −2.50 0.250
Neutral 0.018[±0.16] −2.57 0.147
Note.Properties of the lines of bestfit for our [C II de] ficit measurements. A line of best fit is displayed for each component of the [C II] emission (combined ionized and neutral, ionized, and neutral). We further divide our fits by region type, with afit for all individual regions, a fit for the averaged BtP inner regions and the KINGFISH nuclear regions, and afit for the averaged BtP outer regions and the KINGFISH extranuclear regions.
far-infrared color included in this sample. This holds for both the warmer KINGFISH nuclear and inner BtP regions as well as the slightly cooler outer BtP regions and the KINGFISH extranuclear star-forming regions.
Wefind that the neutral [C II emission traces the total PAH] emission well(Figure6, right panel). We interpret this result as naturally arising from the ubiquity of PAHs in the neutral ISM (e.g., PDRs) and their comparative paucity in ionized portions of the ISM such as H II regions (e.g., Helou et al. 2004).
This result is consistent with [C II being a major cooling] channel, and PAHs providing a majority of the heating through the photoelectric effect, for the neutral ISM. Therefore, in a scenario where ISM heating is balanced by ISM cooling, [C II]Neutralemission should trace PAH emission. As a majority
of the [C II emission in our sample originates in the neutral] ISM(Figure3), the ratio of combined ionized and neutral [C II] emission to PAH feature strength(Figure6, left panel) shows a similar trend with a slight decrease in the ratio of [C II to PAH] emission in regions with the highestnfn(70 mm ) nfn(160 mm ) values. The ratio of[C II emission from the ionized phases of] the ISM to PAH emission feature strength shows a decrease with respect to far-infrared color(Figure6, middle panel). This observed decrease is due to an increasing[C II]Neutralfraction in
the warmer, more actively star-forming regions (i.e., higher
( ) ( )
nfn 70 mm nfn 160 mm ), making [C II]Ionized lower in these
regions. Such a decrease could be due to a fractionally higher absorption of UV photons within H II regions for warmer, more actively star-forming environments as described in Section 4.1.
4.3.[CII] as an SFR Indicator
The top panel of Figure 7 shows star formation surface densities (ΣSFR) as a function of [C II surface brightness]
(S[C II]) for the [C II emission from both the neutral and ionized] phases of the ISM (left), the [C II emission arising from only] the ionized phase of the ISM (middle), and [C II emission] arising from only the neutral phases of the ISM (right). The
SFRs were determined using the hybrid FUV+24μm local SFR indicator(Hao et al.2011; Liu et al.2011; Calzetti2013):
( ) ( ) ( ) ( ) ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ - = ´ - - + m -M L L SFR yr 4.6 10 FUV erg s 6.0 24 m erg s . 9 1 44 1 1
After calculating the SFR using Equation (9), ΣSFR was
calculated by dividing by the deprojected area of the 20″ regions. The area was deprojected by dividing bycos where i isi
the inclination of the galaxy disk.cos was determined usingi
( ) ( ) = - - i q q cos 1 1 10 2 2 2 2
from Dale et al. (1997), where ò is the disk’s ellipticity as
measured by de Vaucouleurs et al.(1991) and q is an adopted
intrinsic axial ratio(i.e., the ratio of the minor axis to the major axis), with a value of q=0.13 for galaxies of morphological class Sbc and later and q=0.2 for galaxies earlier than Sbc (Dale et al.1997; Murphy et al.2018). The same deprojected
area was used to determineS[C II], which is the luminosity of the [C II emission from each region and each separated phase] divided by the deprojected area.
As found in previous studies(Stacey et al.1991; Boselli et al.
2002; De Looze et al.2011,2014; Sargsyan et al.2012; Herrera-Camus et al.2015; Díaz-Santos et al.2017; Smith et al.2017),
there are clear trends with increasing [C II surface brightnesses] indicating increasing star formation. The lines in Figure 7
represent the lines of bestfit to our data for all individual regions as well as the combination of the KINGFISH nuclear regions and the inner BtP regions and the combination of the KINGFISH extranuclear star-forming regions and the outer BtP regions, determined using the method described in Kelly (2007). This
method uses a Bayesian linear regression that takes into account both detections and upper limits. Between 5000 and 10,000 Markov chain Monte Carlo (MCMC) steps through the
Figure 6.Left: the ratio of combined ionized and neutral [C II emission to PAH feature emission strength plotted against far-infrared color. Notice a small decline in the] warmer, more actively star-forming regions at higher 70μm/160μm ratios. Middle: the ratio of [C II emission from only the ionized ISM to PAH feature emission] strength plotted against far-infrared color. Right: the ratio of [C II emission from only the neutral ISM to PAH feature emission strength plotted against far-infrared color.] Notice the sharp decline in this ratio in the ionized emission and the lack of decline in the neutral emission. Markers are the same as in previousfigures.
parameter space are then tested to determine the best-fit relationship. The relationships found for each component of the [C II emission are described using Equation] (11) and the
values displayed in Table7:
( ) ( ) ( ) [ ] S = S + - -- -M m b log yr kpc log erg s kpc . 11 10 SFR 1 2 10 C II 1 2
To test our measurement of SFR, the SFR indicator of Hao et al. (2011) determined using a lower dust attenuation
coefficient, which is identical to Equation (9) but with a
proportionality constant for the 24μm luminosities of 3.89 instead of 6.0, was also applied to each region in our sample. We that find our linear fit parameters have no dependence on the coefficient we use.
The bottom panels of Figure7show the differences between ΣSFRmeasured with the FUV and 24μm hybrid star formation
indicator (Equation (9)) and ΣSFR measured using the
relationships we determined with all the individual regions and using the different components of the [C II emission] (Equation (11) and Table 7). The median value of the
difference forΣSFRmeasured by the summation of the ionized
and neutral[C II emission is] −0.024 dex with a range of 0.80 to−0.36 dex. The range for the differences in ΣSFRmeasured
by the [C II]Ionizedsurface brightness is 1.87 to−0.54 dex with
a median value of−0.058 dex, and for the ΣSFRmeasured by
[C II]Neutral surface brightness it is 0.025 dex with a range of
0.80 to −0.39 dex. We plot these differences (logarithmic ratios) to better illustrate the scatter about our best-fit lines.
The [C II] luminosity is plotted against the SFR for the combined ionized and neutral[C II emission and the [] C II from]
Figure 7.Top: the [C II surface brightness plotted vs. SFR surface density for the combined ionized and neutral [] C II emission from each region] (left), only the [C II] emission from the ionized phase of the ISM(middle), and only the [C II emission from the neutral phase of the ISM] (right). Black lines represent the fits determined using MCMCfitting and blue shaded regions show the full range of lines attempted in the MCMC fitting. Bottom: the difference in the measurements of SFR using FUV+24μm measurements and using the derived combined ionized and neutral [C II surface brightness SFR relationship] (left), [C II surface brightness from only] the ionized ISM SFR relationship(middle), and [C II surface brightness from only the neutral ISM SFR relationship] (right).
Table 7 Linear Fits from Figure7
[C II Component] m b Rms Scatter
All Individual Regions
Ionized+ Neutral 1.04[±0.053] −42.74 0.230
Ionized 0.91[±0.085] −37.11 0.332
Neutral 0.90[±0.050] −37.01 0.269
All Inner Regions
Ionized+ Neutral 1.11[±0.112] −45.15 0.270
Ionized 0.88[±0.180] −35.65 0.405
Neutral 1.02[±0.104] −41.78 0.271
All Outer Regions
Ionized+ Neutral 1.23[±0.185] −50.00 0.152
Ionized 0.90[±0.400] −36.68 0.314
Neutral 0.96[±0.198] −39.55 0.235
Note.Properties of the lines of bestfit for our ΣSFR–S[C II]relationships determined using the method of Kelly(2007). A line of best fit is displayed for each component
of the [C II emission] (combined ionized and neutral, ionized, and neutral). We further divide ourfits by region type, with a fit for all individual regions, a fit for the averaged BtP inner regions and the KINGFISH nuclear regions, and afit for the averaged BtP outer regions and the KINGFISH extranuclear regions.
the isolated ionized and neutral ISM phases in the top panels of Figure 8. In these plots, the LIRGS from the Great Observatory All-sky LIRG Survey(GOALS) are included to expand the range of parameter space covered(Díaz-Santos et al.2017). The LIRGS
in this survey were similarly covered at the[C II]158μm line with PACS on Herschel and at the [N II]205μm line with SPIRE-FTS on Herschel. More information about the observa-tions and processing of these maps can be found in Díaz-Santos et al.(2013) and Zhao et al. (2013,2016). The inclusion of this
sample extends our study to include the more extreme infrared (LIR 1011Le) LIRGS that were part of the GOALS sample. In
addition to the GOALS sample, the handful of high-redshift (z 4) galaxies with [N II]205μm detections are plotted in Figure 8 (Pavesi et al. 2016, 2019; Lu et al. 2017). As
measurements of newere unavailable for these sources, we used
an average value of RIonized=4.0 to determine fNeutral for these
sources. None of the high-redshift sources were included in the line fitting. Similar linear fits were performed on these data and are shown in Figure 8. These trends are described using Equation(12) and the values listed in Table8:
(M -)=m L([ ])( -)+b ( )
log SFR10 yr 1 log10 C II erg s 1 . 12
The bottom panels of Figure8 show the differences between the SFR measured by the FUV+24μm hybrid star formation indicator(SFR(FUV+24), Equation (9)) and the SFR determined
using the relationships found for the [C II luminosity, labeled]
Figure 8.Same as Figure7, but with SFR plotted against the[C II luminosity from the different ISM phases. Orange stars are measurements of LIRGS from the] GOALS survey(Díaz-Santos et al.2017). Gray squares represent the limited sample of published high-redshift (z 4) galaxies from Lu et al. (2017) and Pavesi et al.
(2016,2019) where RIonizedwas set to four because measurements of neare unavailable.
Table 8 Linear Fits from Figure8
[C II Component] m b Rms Scatter
All Individual Regions
Ionized+ Neutral 0.96[±0.036] −39.46 0.229
Ionized 0.92[±0.057] −37.32 0.332
Neutral 0.92[±0.035] −36.03 0.237
All Inner Regions
Ionized+ Neutral 1.05[±0.079] −42.91 0.272
Ionized 1.10[±0.145] −44.32 0.414
Neutral 0.96[±0.075] −39.42 0.270
All Outer Regions
Ionized+ Neutral 0.97[±0.176] −39.86 0.167 Ionized 0.63[±0.182] −25.81 0.235 Neutral 0.88[±0.227] −36.31 0.228 With GOALS Ionized+ Neutral 1.02[±0.022] −41.64 0.309 Ionized 1.02[±0.030] −41.11 0.406 Neutral 0.98[±0.022] −40.05 0.313
Note.Properties of the lines of bestfit for our SFR–L([C II]) relationships determine using the method of Kelly(2007). A line of best fit is displayed for
each component of the [C II emission] (combined ionized and neutral, ionized, and neutral). We further divide our fits by region type, with a fit for all individual regions, afit for the averaged BtP inner regions and the KINGFISH nuclear regions, and afit for the averaged BtP outer regions and the KINGFISH extranuclear regions.
SFR([C II]) (Equation (12) and Table8). We find a median value
of−0.036 dex in the differences between the SFR measured using the hybrid FUV+24μm indicator and the combined ionized and neutral [C II luminosity,] −0.051 dex in the differences between SFR(FUV+24) and the SFR measured by only [C II]Ionized
luminosity, and 0.031 dex in the differences between SFR(FUV +24) and the SFR measured by only the [C II]Neutralluminosity.
The high-redshift sources seem to follow similar trends with greater scatter, potentially due to the large uncertainties in the [N II]205μm detections.
Wefind that the combined ionized and neutral [C II emission] and the[C II emission from only the neutral phases of the ISM] trace SFR as measured by the FUV+24 μm hybrid SFR indicator with a scatter of∼0.23dex. The measured slope of 0.96 [±0.036] for the combined ionized and neutral [C II luminosity–SFR] relation is consistent with the relationship found by De Looze et al. (2014), where a slope of 1.01 [±0.02] was found for a
sample of dwarfs, ULIRGs, AGNs, and starburst galaxies, and the relationship found by Pineda et al. (2014), with a slope of
0.98 [±0.07] for [C II] luminosity within the Milky Way. Using Equation (2), and our result for the SFR measured by
the [C II]Neutral luminosity, we can write an equation for SFR
measured by[C II]158μm and [N II]205μm luminosities:
( ) [ ([ ] ) ([ ] )] ( ) = - -- -M L R L
log SFR yr 0.99 log C II 158 erg s
N II 205 erg s 40.49. 13
10 1 10 1
Ionized 1
This equation has potential to be used in both the local and high-redshift universe without the need for dust corrections.
As the [C II emission from the neutral phases of the ISM] accounts for most of the[C II emission from these regions, it is] expected that the combined ionized and neutral [C II emission] and [C II emission from only the neutral phases of the ISM] follow similar trends, as shown by the similar slopes measured by our lines of bestfit. Although both components of [C II emission] rise with higher star formation surface densities, the [C II] emission from the neutral ISM shows a more tightly constrained relationship than the[C II emission from the ionized ISM. This] increased rms of 0.33 for [C II]Ionized, 0.1 dex above the rms for
[C II]Neutral, is likely due to the sharp decrease in[C II]Ionized/TIR
as a function of far-infrared color. As described in Section4.1, this result could indicate that a large fraction of the [C II emission] from ionized phases of the ISM is not coming from star-forming H II regions, but originates instead in the diffuse ionized ISM (Díaz-Santos et al.2017; Herrera-Camus et al.2018a). The large
scatter in the [C II]Ionized–SFR relationship indicates that any
attempt to use[C II emission as a tracer of SFR must be treated] with caution in galaxies that will have a large fraction of [C II] emission from ionized phases of the ISM, such as high-redshift Lyα emitter galaxies. This conclusion is supported by analysis of the kiloparsec-resolution [C II detections from the SHINING] survey(Herrera-Camus et al.2018a). We also find no difference
in the slopes within error when we separate our regions by location within the galaxy, suggesting that these results hold in a variety of conditions. The inclusion of the GOALS sample raises the slope slightly, which we believe is due to the elevated SFRs of these U/LIRGS, which causes them to fall above the galaxy main sequence(Elbaz et al.2011; Murphy et al.2013).
5. Conclusions
With the recent availability of[N II]205μm detections in galaxies in the local universe from the KINGFISH and BtP
surveys, we are able to distinguish emission from the [C II]158μm line from the ionized and neutral phases of the ISM. The sub-kiloparsec resolution of these [N II]205μm spectral maps makes them an ideal resource for separating [C II emission by ISM phase. Our main conclusions are:]
1. The[C II emission from our sample primarily originates] from the neutral ISM, with an average neutral fraction
[ ]
fC II ,Neutral of 67%. The[C II emission from the ionized] ISM only dominates in a few regions where far-infrared color temperatures are coolest.
2. The trend of decreasing[C II]/TIR as a function of far-infrared color, commonly referred to as the [C II deficit,] is most prominent when only the [C II emission from the] ionized phases of the ISM is considered, and is almost non-existent in the[C II emission from the neutral ISM.] 3. The differences in the behavior of the[C II deficit are] likely due to the majority of the ionized[C II emission] originating in the diffuse ionized ISM. In warmer regions with increased deficit, the FUV radiation required to heat the diffuse ionized ISM is proportionally more absorbed by dust and therefore unavailable to ionize carbon, decreasing the [C II emission we observe from this] phase.
4. The ratio of[C II emission from the neutral ISM to PAH] emission strength is fairly constant in our sample, suggesting that in the neutral ISM gas heating is controlled by PAHs.
5. The ratio of [C II emission from the ionized ISM to PAH] emission strength decreases sharply as a function of infrared color. This result is consistent with a majority of the [C II emission from the warmer regions originating in] the neutral ISM, decreasing the strength of[C II]Ionized.
6. Wefind that the [C II emission from the neutral phases of] the ISM traces the SFR with a scatter of∼0.23dex, while the[C II emission from the ionized phases of the ISM] trace SFR with a scatter of∼0.33dex. The smaller scatter in the neutral[C II]–SFR relationship is inherently tied to the lack of a[C II]Neutraldeficit.
7. We do not find strong dependences on spatial location within the galaxies. However, 85% of the regions sampled lie with 0.25R25, limiting the interpretation of
this result.
The work presented here is limited to the normal star-forming galaxies observed with the PACS spectrometer in the KINGFISH survey. Studies of the[C II emission in AGNs and] LIRGs have found that [C II]–SFR relationships are more scattered in extreme conditions(De Looze et al.2014; Herrera-Camus et al.2015). Despite this increased scatter for
infrared-luminous and accretion-powered environments, there is reason to believe that the SFR–[C II]Neutralrelationship presented here
will hold in a wide variety of environments. Additional measurements of the[C II and [] N II lines in LIRGs produce] similar trends in the deficit behaviors for [C II emission from] the ionized and neutral phases of the ISM (Díaz-Santos et al.
2017). We plan to further investigate the samples presented,
particularly with respect to any trends that may depend on quantities such as metallicity and photoelectric efficiency. Better understanding in detail of the nature of the[C II deficit] in local galaxies, where we can disentangle the contributions from different ISM phases, is critical to interpreting [C II] observations of galaxies at higher redshifts.
